cros: Prevent casual shut down and power save during migration.

chrome/browser/resources/chromeos/login/screen_encryption_migration.js

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 login.createScreen('EncryptionMigrationScreen', 'encryption-migration',
     function() {
   return {
     EXTERNAL_API: [
       'setUIState',
       'setMigrationProgress',
@@ skipping 30 matching lines @@
       var headerBar = $('login-header-bar');
       headerBar.allowCancel = false;
       headerBar.showGuestButton = false;
       headerBar.showCreateSupervisedButton = false;
       headerBar.signinUIState = SIGNIN_UI_STATE.HIDDEN;
     },
 
     /**
      * Updates the migration screen by specifying a state which corresponds to
      * a sub step in the migration process.
-     * @param {number} state The UI state to identify a sub step in migration.
+     * @param {EncryptionMigrationUIState} state The UI state to identify a sub
+     *     step in migration.
      */
     setUIState: function(state) {
       $('encryption-migration-element').uiState = state;
+
+      // Hide "Shut down" button during migration.
+      $('login-header-bar').showShutdownButton =
+          state != EncryptionMigrationUIState.MIGRATING;
     },
 
     /**
      * Updates the migration progress.
      * @param {number} progress The progress of migration in range [0, 1].
      */
     setMigrationProgress: function(progress) {
       $('encryption-migration-element').progress = progress;
     },
 
@@ skipping 29 matching lines @@
 
     /**
      * Updates the string representation of necessary space size.
      * @param {string} space
      */
     setNecessarySpaceInString: function(space) {
       $('encryption-migration-element').necessarySpaceInString = space;
     },
   };
 });